With the advent of the big data epoch, a large quantity of broadband applications and information consumption have emerged, which brings forward new demand for transport, storage, and computation of massive information. Data centers have played a decisive role and as part of the infrastructure of the information society for meeting the demand. Explosive growth of network information drives data centers to develop along direction of super-large size. However, traditional wiring method of direct linking in-between device ports has made wiring in-between switches in super-large data centers extremely complicated.
Complicated and long wiring incurs huge difficulty for system maintenance. Firstly, system maintenance would be extremely difficult in case of change of network connection or of invalid lines. Secondly, dense wiring impacts heat dissipation for racks, as reported by IBM. The Cisco white book further points out that wiring impacts scalability and power consumption of the system, which has become an industry-wide acknowledged bottleneck. Thus, it has become an important research topic to find means for reducing wiring complexity in meeting requirements of ever expanding data centers.
Currently, means for reducing wiring complexity in the industry are divided into the following:
(1) Wireless-Based Wiring
Wireless-based wiring mainly substitutes wireless links for wired links of part or all devices. Such a wiring mechanism poses many problems in actual applications. Firstly, rates of wireless links are far from meeting broadband requirements of the links. Secondly, weak capability of the wireless channels in anti-interference requires extra precision regulating mechanism to prevent interference among the channels, which results in poor device scalability. Thirdly, wireless stations have high power consumption.
(2) Wavelength Division Multiplexing (WDM) Based Wiring
The method uses WDM ring networks to realize logically full wiring among racks, with each pair of the wavelengths corresponding to a pair of racks. The method appears to simplify the network structure and wiring complexity, but in reality, connection to N ToRs would require O(N2) wavelengths. When faced with a scale of tens of thousands of racks, billions of wavelengths would be required, showing that the method has poor scalability.
(3) Arrayed Waveguide Grating (AWG) Based Modularized Interconnection Wiring
AWG based modularized interconnection wiring mainly uses optical fiber wavelength division multiplexing (WDM) techniques and AWG wavelength routing capacity for reducing wiring complexity. An N×N AWG interconnection network is constructed via a three-level network consisting of (N/r)2 interconnected r×r AWGs, with wiring number being reduced to O(N2/r). Hence, the employment of AWG modularized interconnection networks in data center networks reduces wiring number while at the mean time provides sufficient communication bandwidth. However, as the scale of the data center further increases, the number of ports r of the r×r AWGs increases, which still results in scalability issues: firstly, the requirement of the networks for a large number of optical terminals with various wavelengths leads to high cost for system deployment; secondly, the AWGs with many ports produce substantial intraband crosstalk; thirdly, the difficulty of AWG processing increases.